The present invention relates to a positioning method and apparatus for, e.g., a semiconductor exposure apparatus which is required to accurately detect the height and surface position of an object and, more particularly, a positioning method and apparatus suitable for accurate continuous detection of wafer surface position and tilt, which is required in a slit-scanning exposure apparatus.
Recently, the memory chip geometries are gradually increasing upon reflecting improvement in resolution line width of exposure apparatuses or an increase in cell size and memory capacity. For example, the first generation of 256M DRAMs has a chip size of about 14.times.25 mm. With this chip size, a reduction projecting exposure apparatus (stepper) which is currently used as a critical layer exposure apparatus and has a 31-mm diameter exposure area can expose only one chip per cycle, and the efficiency is low. Hence, the exposure apparatus is required to expose a larger area.
As a large-area exposure apparatus, conventionally, there is a rough layer exposure apparatus used in manufacturing a semiconductor device or an exposure apparatus for manufacturing a wide-screen liquid crystal display device such as a monitor, for which high throughput is required. The latter, i.e., the exposure apparatus for manufacturing a wide-screen liquid crystal display device is a so-called slit-scanning exposure apparatus using mask-wafer relative scanning. This exposure apparatus uses a reflection projecting optical system. An image of a pattern on a mask is formed on a wafer through a concentric reflecting mirror optical system. Arcuated slit illumination light is used for illumination. The mask is linearly scanned with respect to the slit, thereby fully exposing the entire area.
In the exposure operation, a wafer or a glass plate substrate to which a photoresist as a photosensitive agent is applied must be focused with respect to a mask image. In the above exposure apparatus, since the surface to be exposed is sequentially set on the best imaging plane of the projecting optical system, surface position measurement for detecting height and tilt and correction driving such as autofocusing/autoleveling are continuously performed even during scanning and exposure. As the surface position detection means for detecting height and tilt, an optical sensor is used. For example, an oblique incident optical system projects a light beam onto the surface of the wafer as a photosensitive substrate obliquely from the upper side, and the light reflected by the substrate is guided to the sensor to detect height and tilt on the basis of a positional shift on the sensor. Alternatively, a gap sensor such as an air microsensor or an electrostatic capacitance sensor is used. The heights of a plurality of substrates which are being scanned are measured by using such a sensor. The height and tilt correction driving amounts when the measurement area passes the exposure slit are calculated from the measurement value, and correction is performed.
However, as the circuit pattern continues to shrink in feature size and the NA of the reduction projecting system increases, the allowable focal depth in the circuit pattern transfer process becomes smaller. Currently, in the exposure apparatus for the rough process, an allowable depth of 5 .mu.m or more is ensured, and measurement errors contained in the measurement values obtained by continuous measurement during scanning exposure or a step difference in a chip can be neglected. However, to cope with a 256M DRAM, the allowable depth is 1 .mu.m or less. If the concept of the slit-scanning exposure apparatus currently in use is directly diverted to manufacture a 256M DRAM, and only the projecting optical system is improved, measurement errors or a step difference in a chip becomes large and cannot be neglected.
An example of measurement error generation in scanning is described in Japanese Patent Laid-Open No. 6-260391. In this prior art, a difference z in height/tilt between a wafer surface and an imaging plane, and the height/tilt of the stage at that time are detected at a measurement position before the exposure area. As the characteristic feature of this control method, when the detected area reaches the exposure area, the height of the area is set at a value obtained by adding the difference z to the detected height of the stage. To obtain the control amount, the focusing measurement values are subjected to mean processing to relax the influence of unevenness on the wafer surface. As for stage position, however, an immediate measurement value is used because there is no concept of storage time.
Assume that the correction amount at the periphery is 2 .mu.m, and correction is performed in 50 msec. The height and tilt are continuously measured even while the stage is being driven by 2 .mu.m in 50 msec. Assuming that the storage time is 5 msec, and the correction driving pattern is linear, the moving amount along the Z-axis in the storage time is 0.2 .mu.m. As described above, a storage time error is contained in measurement during driving, and a correction error around 0.2 .mu.m at maximum is generated in immediate control.
In the exposure sequence of the conventional stepper, exposure is started after correction of focusing (height and tilt) at the exposure position is completed. That is, serial processing is performed. For this reason, exposure can be performed in a stationary state after focusing measurement and driving are completed. However, the slit-scanning exposure sequence is performed in parallel processing. That is, exposure is performed even during focusing measurement. Hence, the two processing operations, focusing measurement driving and exposure, must be simultaneously accurately performed. Any positional and time shifts between the focusing measurement position and the exposure position, and the height position of the leveling stage in focusing measurement, must be carefully monitored.
In slit-scanning, focusing measurement, correction driving, and exposure are simultaneously done during scanning. For this reason, the focusing measurement position to be detected by the surface position detection means must be set in front of the exposure position in the scanning direction. This is because focusing driving correction must be ended before the focusing measurement point reaches the exposure slit position.
When a storage sensor is used for focusing measurement, the mean of measurement values in the storage time is obtained as measurement data. The exposure apparatus always vertically moves the leveling stage for exposure. If the height and tilt positions of the leveling stage in focusing measurement are simply measured and corrected, accurate wafer height and tilt correction cannot be realized.